1. Field of the Invention
The present disclosure relates generally to methods and systems for borehole telemetry in surveys relating to subterranean formations. More specifically, some aspects disclosed herein are directed to methods and systems for transmitting data between a borehole tool and a surface system over an electrical cable utilizing multiple data communication channels and data that is pre-coded at the transmitter to improve the reliability of data transfer and achieve a gain merit, in particular, in difficult environments such as borehole logging.
2. Background of the Related Art
Accurate and rapid collection and distribution of geophysical property data is a key to successful exploration and production of petroleum resources. Based on data such as electrical and nuclear properties collected in a wellbore, as well as the propagation of sound through a formation, geophysicists make an analysis useful in making many important operational decisions. The analysis includes determination of whether a well is likely to produce hydrocarbons, whether to drill additional wells in the vicinity of an existing well, and whether to abandon a well as being unproductive.
Measurements of the type described herein are also useful in the fields of CO2 sequestration, development of methane hydrate deposits, water reservoir monitoring, earthquake monitoring, and monitoring for reservoir delineation, among other applications that are known to persons skilled in the art.
Geophysicists may also use wellbore data to select where to set casing in a well and to decide on how to perforate a well to stimulate hydrocarbon flow. One method of collecting wellbore geophysical properties is by way of well logging. In well logging, a logging tool (also often referred to as a sonde) is lowered into a wellbore on an electrical cable, the wireline. The logging tool is an electrically powered measurement device that may, for example, collect electrical data, sonic waveforms that are propagated through the surrounding formation, or radioactivity counts. These measurements are usually converted to a digital form and transmitted on the wireline. Systems for transmitting data from the borehole logging tool to a data acquisition system over a wireline cable are known as wireline telemetry systems.
Commonly owned U.S. Pat. No. 5,838,727 of Schlumberger Technology Corporation discloses one wireline telemetry system. This wireline telemetry system transmits data from the logging tool to the surface computer using a single carrier frequency. Wireline cables are primarily designed for mechanical properties. A modern oil well may be drilled to a depth of in excess of 30,000 feet. The cable must be able to sustain the tension generated from the weight of the logging tools and the weight of the lengthy cable itself.
As discussed above, logging and monitoring boreholes has been done for many years to enhance and observe recovery of oil and gas deposits. In the logging of boreholes, one method of making measurements underground includes attaching one or more tools to a wireline connected to a surface system. The tools are then lowered into a borehole by the wireline and drawn back to the surface (“logged”) through the borehole while taking measurements. The wireline is usually an electrical conducting cable with limited data transmission capability. Similarly, permanent monitoring systems are established with permanent sensors that are also generally attached to an electrical cable.
Demand for higher data rates for wireline logging tools and permanent monitoring systems is growing rapidly because of higher resolution sensors, faster logging speeds, and additional tools available for a single wireline string. Although current electronic telemetry systems have evolved, increasing the data transmission rates from about 500 kbps (kilobits per second) to 2 Mbps (megabits per second) over the last decade, data rates for electronic telemetry systems are lagging behind the capabilities of the higher resolution sensors. In fact, for some combinations of acoustic/imagining tools used with traditional logging tools, the desired data transmission rate is more than 4 Mbps.
In addition, while higher data rates are desirable, many tools in current use would have to be completely reworked or replaced to incorporate new data transmission technologies. It would be desirable to facilitate faster data transmission rates with minimal changes to existing tools and equipment.
As a consequence of a need for higher data rates problems in data transmission arise such as degradation of data quality. Therefore, it would be desirable to transfer data between downhole and surface tools with improved signal to noise ratio (SNR) so that the quality of the transferred data is not compromised.
From the foregoing it will be apparent that there is need for improving data throughput in downhole telemetry systems, and to have a telemetry system that is more adaptable to the specific characteristics of the cable that is used to transfer data.